To achieve a good performance of digital communication systems the carrier recovery is very important. In wireless communication systems, the sensitivity to frequency offset is one of the major issues for practical application. If the carrier frequency offset will be not properly compensated, it would result in a shift of the sub-carrier indices and thus produce inter-carrier interference in multi-carrier system. In single carrier system, the frequency offset will cause instability of the time-domain equalizer.
The carrier frequency offset is mainly caused by transmitter-receiver oscillator instabilities and Doppler shift in mobile environment.
There are a lot of existing methods to estimate the carrier frequency offset. A method uses a correlation to estimate the carrier frequency offset, but its estimation range is not large enough in some situations. Another method uses a FFT (Fast Fourier Transformation) to estimate the carrier frequency offset, but the accurate position of the training sequence need to be known before estimation.
A PN-based structure (PN: Pseudo random Noise) is adopted in terrestrial digital TV broadcasting standard in China. There are three kinds of frame structure in this system as shown in FIG. 7. There are shown three frame structures, which are used in the terrestrial digital TV broadcasting standard (TV: Television Standard) in China.
The frame structures starts with a frame head of different length being followed by data having length of 3780 data symbols.
The first frame structure (a) uses an 8th-order m sequence to generate PN. The first 82 symbols and the last 83 symbols in frame head of the first frame structure (a) are pre-cyclic and post-cyclic of the PN sequence. In 225 consecutive frames, the PN has different phase in different frames.
The second frame structure (b) uses a 10th-order m sequence to generate PN and uses the first 595 bits of this PN sequence as the frame head.
The third frame structure (c) uses a 9th-order m sequence to generate PN, the first 217 symbols and the last symbols in frame head being pre-cyclic and post-cyclic of the PN sequence. In 200 consecutive frames, the PN has different phase in different frames.
Considering the complexity of the system, the method should estimate the carrier frequency offset no matter what the structure of the frame head is.
FIG. 8 shows a block diagram of carrier recovery in a digital system. RF signals s (RF: Radio Frequency) are send from a transmitter 1 via antennas and an airborne link V to a receiver comprising a tuner 2. The tuner 2 receives the RF signal s from the antenna and converts it to the expected IF signal IF. An in-phase and quadrature demodulation (iqm) 3 comprises a frequency spectrum shifter and some filters. The iqm module 3 moves the IF signal IF to baseband. After the iqm module 3, a carrier recovery module (cr) 4 is followed, which is in the head of other demodulation block in the baseband. The following block's performance would be affected when a residual frequency offset is large. In PN-based systems, the correlation value of PN sequence would be used for frame synchronization and timing recovery module (tr) 5, but existence of the large residual carrier frequency offset would cause very bad correlation performance. Even no peak appears at all. And inter-carrier interference (ICI) would be imported when a fractional frequency offset of sub-carrier exists. Further, such arrangement comprises a channel estimation and equalizer module (che/eq) 6 output of which is coupled to the carrier recovery module 4 and to a forward error correction module (fec) 7.
According to such state of the art devices and methods critical points for carrier recovery are widen of given estimation ranges and an increase of the estimate precision.
FIG. 9 shows a block diagram of devices executing a known method of carrier recovery. Input data id or an input signal are fed to a rotator 11. The block of the rotator 11 is used to compensate a carrier frequency offset for the input signal. Signal output out of the rotator 11 are inputted into a selector 12. The selector 12 is constructed and/or controlled to find a coarse position of PN sequence to be used in following estimator modules 13, 14 for estimation of frequency offset. A coarse estimator 13 of these estimator modules 13, 14 adapts a squared correlation result of its input signal to estimate the carrier frequency. After such coarse estimation, a residual carrier frequency offset is within a small range. The fine estimator 14 of these estimator modules 13, 14 correlates its input data with local PN sequence to estimate the residual frequency offset. This method uses the correlation results to estimate the frequency offset in each step. A state control 16 controls a switch 15. The switch is arranged to forward signal either outputted from coarse estimator 13 or outputted from fine estimator 14 to input of a low pass filter (LPF) 17. Data or signal filtered by the low pass filter 17 are inputted into a numerical controlled oscillator (NCO) 18. Signal or data outputted out of this NCO 18 are inputted into the rotator 11.
A main drawback of this method is that result is not stable when estimating a large frequency offset. Further, the residual frequency offset is not small enough when the PN sequence has different phase in different frames within a certain convergent time.
It is an object of the invention to provide an other carrier recovery device for pseudo random noise based systems and to provide a carrier recovery method for pseudo random noise based systems, especially, being able to estimate large frequency offsets.
Especially, it is an object is to find a way using PN (Pseudo random Noise) to estimate the carrier frequency offset in a wide range, no matter whether it is multi-carrier systems or single-carrier systems. Preferably, method and device solving such object should be able to work without exact information of the position of the training sequence.